Field of the Invention
The present invention generally relates to the recording of information signals onto plate-like, metallic recording carriers.
For nearly a century, sound has been recorded in analog fashion, i.e., by electromechanically cutting information signals into the surface of a recording carrier and then repeatedly copying the recording to produce finished phonograph discs. In recent years, however, digital sound and data recording have gained significance. In digital recordings, information is applied to a recording carrier by complicated optical and chemical processes and then is read from the carrier by contactless scanning. Digital recordings are exemplified, for example, by so-called CD (compact disc) recordings which are noted for their good acoustic properties.
Although CD recordings are substantially easier for users to handle than conventional phonograph records, manufacturing of a CD recording is more complicated than manufacturing conventional recordings. For example, the step of "cutting" signal information onto a compact disc can only be carried out in "super-clean areas" which substantially limit the risk that dust particles will impair the manufacturing process.
In the conventional manufacture of CD recordings, a glass plate is first polished, cleaned, dried and then provided with a photosensitive coating. After cutting digital signal information onto the prepared plate with the aid of a laser beam, a so-called glass "master" is prepared which carries the signal information. Then the surface of the glass master is made electroconductive by silvering and it is used to take a metal "negative" from which, in turn, a metal "positive" is taken. The metal positive then serves as a so-called "mold mother", from which "stampers" are taken. The stampers are used as raw material for pressing of CD discs, which is similar to the pressing of analog discs. After pressing, the CD discs are coated with a thin aluminum layer and a protective layer.
In recent years, conventional methods of producing analog sound recording have been improved. For example, it is no longer required that analog sound information be cut into a lacquer film but, instead, sound information can be directly cut into metallic recording carriers. This newer method of recording sound is sometimes referred to as direct metal mastering (DMM) and has the benefits of providing high dimensional stability of the cut grooves and of eliminating several intermediate manufacturing stages.
As further background to the present invention, it should be understood that successful attempts have been made to use mechanical recording techniques to cut signals of substantially higher frequency than audio frequencies into recording carriers. Such cutting of information signals into recording carriers was performed in a purely analog form; i.e., the recording was in the form of a continuous spiral groove which could be read out by a mechanical scanner. Such recording processes are sometimes referred to as TED video disc processes.
Recording of digital information signals, however, precludes the recording of a continuous groove on a recording carrier. Instead, in digital recording in the CD format, for example, information signals are formed onto information carriers as individual indentations having substantially constant depth and width, but different lengths and spacings. When reading such recordings, a transition between an indentation and the surface of the recording media, or between the media surface and an indentation, is detected and interpreted as an information signal. That is, in the CD format, information signals on a recording media represent a sequence of binary signals which are substantially free from mean values.
In practice, digital recordings of information signals are read by laser beams that detect transitions between the surface of the information-carrying layer and indentations. More particularly, laser beams are virtually completely reflected when the beams strike the recording media surface between indentations but are virtually completely absorbed when directed into an indentation. Absorption of laser beams at indentations result from diffraction, which effectively extinguishes any reflection of the beam. For acceptable operation, the depths of the indentations must be approximately constant and the angles at the sides, or flanks, of the indentations must be maintained relatively exactly. Because the depth of indentations in CD recording is only about 0.1 microns, the manufacturing processes are complex to meet these precise requirements.